1. Field of the Invention
This invention relates generally to the field of batteries and more particularly to pressure release features incorporated in battery cells to avert explosion. The invention is particularly applicable to lithium sulphur dioxide battery cells but may be utilized on other types of battery cells where similar problems occur.
2. Description of the Prior Art
The lithium sulphur dioxide types of batteries are well-known in the art and are most useful in application requiring long shelf life and relatively high current output over brief time spans. Because of their properties, such batteries have been used for powering emergency locator transmitters, personal distress flashers and disposable flashlights. For applications where intermittent high drain is demanded, such batteries are also useful in computer memories, smoke alarms, and photoflash equipment, pingers, transponders and electronic clock standby power. Such batteries also have superior low temperature performance characteristics which makes them useful in arctic hardware, remote sensing equipment and in emergency communications equipment. For their size and weight such batteries store a great amount of energy.
The conventional lithium sulphur dioxide batteries are not without their disadvantages. The cells usually are completely sealed so as to avoid the release of sulphur dioxide which is a very noxious gas. In the event of a short circuit or heating of the battery beyond some predetermined temperature, pressure can build up within the cell and it can eventually explode with the force of a pipe bomb. To avert such possibility, pressure actuated vents or rupture discs are frequently employed in this and other types of battery cells to relieve internal pressure without explosive force. An example of the type of vent employed may be as illustrated in the Patent to Amthor U.S. Pat. No. 3,664,878 entitled "Pressure Relief Vent Valve for a Fluid Type Sealed Container Especially an Alkaline Galvanic Cell". Numerous other examples of vents or rupture discs can also be found in the patent art.
Reported accidents with present commercially available batteries have led to the conclusion that none of the current methods used to vent batteries of the lithium sulphur dioxide type are completely safe. In fact, their performance has become so unpredictable that the Ministry of Transport of Canada has ordered their removal from all emergency locator transmitters in light aircraft.
An examination of the results of explosions that have occurred have led to the conclusion that such explosions could be attributed to two possible causes. Both of these causes are traceable to failure of the mechanical vents in existing cells. One cause is due to overheating which occurs due to a short-circuit, whether internally or within a mechanism to which the battery is connected. In a lithium sulphur dioxide cell all the energy capacity in the cell is sufficient to cause internal heating and raise the temperature within the cell to in excess of 180.degree. C., which is the melting temperature of lithium.
The second cause of explosion is attributable to external heating of the battery cell due to an external fire or due to flaring of an adjacent cell. The provision of some sort of venting means is imperative to avert such explosions. Because of the noxious nature of the sulphur dioxide within the battery, care must be taken to make sure that the batteries do not vent at normal temperatures and under normal usage. At the same time, since the explosive temperature of 180.degree. C. is so low, it has been very difficult to design mechanical vents or rupture discs that will always open between the ranges of temperatures that are considered normal and the explosive temperature of 180.degree. C.